A drill chuck such as described in U.S. Pat. No. 4,836,563 for use with a tool having a shaft typically has a chuck body centered on and rotatable about an axis and an adjustment body centered on the axis, rotatable about the axis on the chuck body, and axially fixed on the chuck body. One of the bodies is formed with a plurality of angularly equispaced jaw-guide passages centered on respective jaw axes all lying on the surface of an imaginary cone centered on the axis. The other of the bodies is formed with a screwthread centered on the axis and exposed in the guide passages. Respective jaws displaceable in the passages along the respective axes are each formed with a row of teeth meshing with the screwthread so that relative rotation of the bodies in one direction moves the jaws radially inward and axially forward and opposite relative rotation moves the jaws radially outward and axially rearward.
Such a chuck is often set up to be self-tightening and is also provided with a locking or holding ring that is axially and limitedly angularly displaceable on the chuck body and engageable with teeth on the adjustment body to limit its rotation. A spring is braced between the locking ring and the chuck body so that, if the jaws loosen, the spring will advance them.
In order that the locking ring not interfere with the self-tightening action, it is necessary that the locking ring be limitedly rotatable on the chuck to follow the rotation of the element that rotates as the chuck self tightens. Two end stops are provided as described in U.S. Pat. No. 4,773,657 and a spring arrangement that normally returns the locking ring to the so-called starting end position corresponding to the position from which it will move as the chuck self tightens. To this end the locking ring is formed with two axially extending short and long grooves and the chuck body has a head that projects into them. In one end position the head is in the long groove so that teeth on the front end of the locking ring engage the adjustment sleeve and prevent it from rotating and in the other position the head is in the short groove and the locking-ring and adjustment-sleeve teeth do not engage. The locking ring can be pulled back against the force of a spring to allow the position to be selected.
In my earlier German patents 3,432,918 and 3,903,443 I describe another such system where the self-tightening movement is limited in order to prevent the jaws from biting too deeply into the tool. These end positions can be defined by a pin engaging radially from the holding sleeve into an angularly elongated slot of the drill spindle on which the chuck body is mounted. Alternately two teeth on the holding ring that coact with planar faces of the chuck body can be used.
Such arrangements are workable, but require that the user operate them very carefully. They must be manipulated carefully, with specific axial and rotary movements in a specific sequence, in order to ensure that the chuck is locked and in self-tightening mode when released. If the holding ring is not, for instance, twisted against the tightening direction before it is released, the tightening spring is not loaded and there will be no subsequent self-tightening action.